Project Summary/Abstract The proposed research project addresses the public health concern of heavy drinking among college students by developing and testing an innovative mobile-based application to target the specific dangerous drinking behavior known as prepartying, a risky behavior practiced by upwards of 85% of college students. Prepartying involves fast-paced heavy drinking in residence halls or off-campus residences prior to going out for the night where students inevitably drink even more. Findings from over 80 prepartying studies suggest the behavior is perhaps the riskiest known college drinking behavior, leading to consequences during the prepartying event, to continued drinking and consequences after prepartying events, and to greater global levels of drinking and alcohol-related problems. Yet the lack of interventions directly targeting prepartying represents a major gap in the college student drinking research field, as such an approach could address and reduce the behavior known to be the greatest risk. For the proposed study, we will develop and test a prepartying-specific brief mobile-app intervention that is intended to help students reduce their prepartying behavior. We first develop the intervention content based on theory and research supporting mechanisms of change in brief interventions with college students and document normative drinking information from 500 college students for inclusion in the intervention content. We then beta test the intervention with a sample of 14 heavy drinking college students. Focus group feasibility and acceptability feedback will inform the final intervention content, as well as help us refine study procedures. We will then pilot test the mobile-based intervention in a randomized controlled trial of 500 college students who preparty frequently (n = 250 intervention, n = 250 attention control) and determine the efficacy of the intervention on (1) preventing heavy consumption levels and consequences during and after prepartying in the immediate term (i.e., pre-intervention two weeks to post-intervention two weeks), and on (2) reducing students? global levels of heavy drinking and consequences one and three months post-intervention. Lastly, we will evaluate hypothesized moderators known to contribute to greater risk during prepartying; mainly, that female students, students under age 21, heavier drinkers, and those motivated to change their drinking will benefit the most from the intervention. We will test a hypothesized effect that changes in prepartying drinking post-intervention (baseline to one-month) will explain changes observed in global drinking and consequences from baseline to three-month post-intervention. This prepartying-specific approach expands on global interventions and improves upon promising event-specific approaches that only focus on infrequently occurring events (e.g., 21st birthdays, spring break) by targeting the popular behavior that precedes multiple drinking contexts and directly leads to further drinking and problems. By intervening at the potential source of concern, this first-ever prepartying intervention has the potential to greatly change the culture of heavy college drinking.